1. Field of the Invention
The present invention relates to improved analytical techniques and means for reproducibly analyzing trace quantities of a wide range of volatile organic compounds and the application of such techniques to the detection of metabolic disorders, flavor and aroma analyses, air and water pollution analyses, petroleum exploration, etc.
2. Description of the Prior Art
The introduction of gas chromatography and detector technology has provided analytical techniques especially suitable for trace analysis of organic substances. A mass spectrometer can be considered to be a specific detector for a gas chromatograph, and its performance can be adjusted according to the needs of the analysis, thus providing an excellent fingerprint or profile of a chemical substance. This technique lends itself to computerization with all its inherent advantages. For example, files containing some 300,000 organic compounds are available even for small laboratories by a suitable instrumental interface to a time-shared computer. In principle, even very complex samples can be analyzed on a routine basis. However, development of reproducible sampling, transfer, storage and desorbing procedures is essential for such analyses.
Techniques presently available for analysis of volatile organic components of body fluids are based on extraction with a suitable solvent, distillation, headspace gas technique, and, in the case of solids, direct head adsorption. Headspace gas techniques are a true representation of volatile material, since each component present in a liquid characteristically contributes to the total headspace gas according to its own vapor pressure at a given temperature. In terms of speed, reproducibility, and true volatile composition, this technique is superior to the others.
Since biological, air, and water samples often contain very complex volatiles consisting often of several hundred compounds, high resolution open tubular capillary columns are very useful; although conventional packed columns may also be used for certain analyses. To make full use of the potentials of high resolution capillary columns, the gas sample should be introduced as a sharply defined plug. Unfortunately, where the sample contains only trace quantities of organic volatiles, the sensitivity of detectors presently available for this kind of analysis falls short by several orders of magnitude, unless volatiles are concentrated. This is true in the case of volatile organic compounds in air and tobacco smoke and in the headgases of petroleum, water, foods, beverages, tissue cultures, and the body fluids.
Previous methods for concentrating traces of organic volatiles include the use of cold traps, freezing out in short tubes, enrichment in relatively short pieces of capillaries or small bore packed columns directly attached to the injector port, and cryogenic injection directly into the capillary column. Freezing is less desirable for samples containing excess moisture, since the water would tend to remove the volatiles as it freezes out. Cryogenic injection is usable only for relatively concentrated samples because the total volume of the sample including the water must pass through the system at a relatively low flow rate.
There has been a need for a relatively simple, fast, inexpensive and reproducible method for detecting diseases, determining the progress of disease, and measuring effectiveness of treatment of disease in humans and animals, particularly a method that lends itself to massive screening for disease.
There has also been a need for a relatively simple, fast and reproducible method for analyzing organic volatiles in closed environments, such as spacecraft, submarines, high altitude airplanes and other enclosures.
A further need is sampling and analysis for forensic purposes, such as identification of objects, marking or providing a "signature" to products by adding some compounds, which cannot be masked from detection.
Air pollution presents additional needs. At the present time, transportation largely depends on vehicles using combustion engines which emit large amounts of substances in the air which effect human environments. Low molecular weight olefins and aromatics in automobile emissions are known to participate in the petrochemical formation of smog. The high molecular weight end of the spectrum of substances emitted are potentially harmful compounds, such as polynuclear aromatic hydrocarbons and heterocyclic compounds, some of which are known to be potent carcinogens.
The majority of people living in industrialized areas are forced to inhale many of these substances daily in varying amounts and changing composition. It has been established that air pollution in general is responsible for a wide variety of pathological conditions involving the respiratory tract, and the pulmonary system. Except for a few isolated cases, little is known about the nature of the chemicals responsible and no information is available on the synergistic and long-term effects.
Some of the reasons for the lack of data on volatile organic compounds in air are due to the difficulties encountered in sampling and analysis. Combustion processes are incomplete and lead to a variety of products of great complexity. Several hundred compounds, therefore, must be analyzed in concentration ranges which can differ by several orders of magnitude.
The problems in analysis of air are similar to the ones encountered for volatiles in body fluids; however, sampling requirements are different for air pollution. Besides fixed gases, water constitutes the major component of air. This leads to considerable difficulties of both sampling and analysis. The organic materials to be analyzed in air are present at a concentration level of 7 to 10 orders of magnitude lower than water. These facts have hindered successful analysis with prior art techniques.
In general, the same problems exist in sampling and analysis of organic compounds in petroleum exploration, water pollution, flavor and aroma analyses, tobacco smoke analyses, and the like. In this connection there is a need for sampling, transfer, storage and analysis which lends itself to the detection of ultratrace quantities of volatiles, such as sampling at a remote place and bringing back a small enriched sample for analysis at a central location.
Some desirable features of a good sampling and analysis system are (1) enriched concentration of volatiles from a large sample, for example air, with minimum interference of moisture, (2) complete collection in the volatility range considered, (3) ability to simultaneously collect multiple samples, (4) storage and transportation of sample without loss or change in composition for analysis at a later time, and (5) quantitative and unaltered regeneration at time of analysis.
In order to be useful for practical applications in an average laboratory, the sampling procedure should also allow samples to be taken with a minimum of interference; the speed of sampling should be sufficient to follow rapid compositional changes; regeneration should be done without intermediary steps, such as extraction; and ordinary laboratory personnel should be able to handle the collection or sampling apparatus.
Presently available sampling technology include cryogenic techniques; at subambient temperatures with solid adsorbents, or conventional gas-liquid chromatography (GLC) packings; at room temperature with solid adsorbents or conventional GLC packings with heat desorption; and at room temperature with solid or chemically bonded stationery phases to adsorbents with liquid extraction. While these sampling techniques do have certain advantages, they do not have all of the desirable features previously mentioned. For example, in cryogenic sampling techniques, there are difficulties in regenerating the sample, especially for high molecular weight compounds. Also, there is strong interference by excess water; it is relatively expensive; and there are losses due to fog formation. With adsorbents at subambient temperatures there is interference from water, the sample size is limited (depending upon the adsorbent) recovery is incomplete, there are viscosity problems with GLC packing, and artifacts are quite common. With adsorbents at room temperature there is a loss of low molecular weight compounds to various degrees (depending on the nature of the adsorbent), recovery is incomplete (depending on the adsorbent), surface reactions are more likely than with many other techniques, excessive heat is necessary for very active adsorbents, flow rate is restricted especially with GLC packings, and there are decreasing efficiencies of recovery with high molecular weight compounds. With sampling techniques utilizing adsorbents at room temperatures, much time is consumed, the sample is diluted with extractant (which is difficult to remove without loss), secondary reactions are possible, skill is required and automation is difficult.
Porous polymers used as adsorbents prior to the present invention were not entirely satisfactory. For example, a porous polymer of styrene and divinylbenzene, sold under the trademark Porapak P, was limited to 230.degree. C. for desorption, and at 200.degree. C. the bleed from this adsorbent was considerable, producing artifacts during analysis. The desorption temperature limit prevents effective desorption of higher molecular weight volatile organics.
Another adsorbent used is a carbon molecular sieve (Carbosieve) prepared by thermally cracking polyvinylidene chloride. It has great surface area and high temperature stability; however, temperatures well over 400.degree. C. are needed to desorb organic volaties, which causes pyrolysis of some of the compounds. Also, extreme care must be taken in its regeneration, handling, and storage.
Another adsorbent used is activated charcoal; however, many of the adsorbed volatiles are difficult to recover because of irreversible adsorption and surface reactions.
In prior art analyses, samples are customarily desorbed prior to injection into the chromatograph, and it is necessary to utilize syringe injections with their attendant disadvantages.
In prior art chromatographs it was customay to use open tubular columns, as disclosed by M. J. E. Golay in U.S. Pat. No. 2,920,478, Jan. 12, 1960. The majority of all open columns is made of stainless steel, although glass is sometimes preferred because of the reaction of stainless steel with labile organic compounds. However, there are many problems in the fabrication and handling of glass columns.
An adsorbent used in one aspect of the present invention is a polymer of 2,6-diphenyl-p-phenylene oxide (PPPO) sold commercially under the registered trademark TENAX GC. This polymer was developed by AKZO, Research and Engineering N.V., Arnhem, The Netherlands. Technical Bulletin No. 24 of Applied Science Laboratories, Inc., State College, PA., the United States distributor, states that the polymer is useful as a polymer column packing material for the separation of high boiling polar compounds such as alcohols, polyethylene glycol compounds, diols, phenols, mono and diamines, ethanolamines, amides, aldehydes, and ketones. Several articles on this porous polymer by R. Van Wijk, of AKZO, supra, relate to its use as a column packing material and various applications in gas chromatography. However, prior to this invention Tenax GC was never used or even suggested for use as an adsorbent for the collection and desorption of volatile organic compounds.